Gene expression profiling gets to the root of human hair follicle stem cells

Abstract

Hair follicle stem cells sustain growth and cycling of the hair follicle and are located in the permanent portion of the follicle known as the bulge. In this issue of the JCI, Ohyama et al. report the characterization of global gene expression patterns of human hair follicle stem cells after their isolation using sophisticated laser capture techniques to microdissect out bulge cells. They discovered a panel of cell surface markers useful for isolating living hair follicle stem cells, a finding with potential therapeutic implications since isolated stem cells in mice can generate new hair follicles when transplanted to other mice. The findings of Ohyama et al. validate the use of the mouse for studying hair follicle biology but also underscore critical differences between mouse and human stem cell markers. In particular, CD34, which delineates hair follicle stem cells in the mouse, is not expressed by human hair follicle stem cells, while CD200 is expressed by stem cells in both species. Ultimately, this information will assist efforts to develop cell-based and cell-targeted treatments for skin disease.

Figure 1

Hair follicle cycle. Cyclical changes in hair follicle growth are divided into different stages, referred to as anagen, catagen, telogen, and exogen. Hair follicle stem cells localize to the bulge at the bottom of the permanent follicle at the site of arrector pili muscle attachment. During anagen, rapidly proliferating progenitor cells in the bulb generate the hair and its surrounding inner root sheath. The onset of catagen is marked by cessation of proliferation and apoptosis of the epithelial cells below the bulge. The mesenchymally derived dermal papilla survives catagen and moves upward to abut the lowermost portion of the bulge, which then forms the secondary germ at its base, during telogen. Ill-defined mesenchymal-epithelial interactions likely involving BMP and WNT signaling are thought to signal anagen onset and the generation of the new lower follicle and hair shaft. As the new hair grows in, the old hair is shed during exogen. The duration of each stage varies depending on the type, site, and genetic programming of the follicle.